A photocatalyst for degrading doxycycline and its preparation method

Abstract

The present invention relates to a photocatalyst for degrading doxycycline and a preparation method thereof. The preparation method includes: S1. Synthesis of MIL-68(In): Dissolve In(NO3)3·5H2O in DMF and completely dissolve it under ultrasonic waves; then add terephthalic acid and dissolve it with ultrasonic waves; react in an oil bath for 40 min, cool and centrifuge, and wash the precipitate with ethanol to obtain MIL-68(In); S2. Synthesis of In2O3 / Ag3PO4: Dissolve MIL-68(In) and Ag3PO4 in water, and dissolve Na2HPO4 in water. Mix the two solutions and stir for 2 h, wash, and calcine for 3 h after vacuum drying to obtain the photocatalyst. The research results of the present invention have guiding significance for the design and development of highly efficient visible-light-responsive photocatalysts and have market application prospects in environmental protection.

Description

Technical Field

[0001] The invention relates to a photocatalyst, in particular to a photocatalyst for degrading doxycycline and a preparation method thereof. Background Art

[0002] At present, due to the excessive pursuit of economic benefits, the use of antibiotics in animal husbandry and aquaculture is becoming more and more common, which inevitably leads to a large amount of antibiotic residues in soil, surface water, drinking water and animal-derived food. Doxycycline is a tetracycline antibiotic commonly used to fight bacterial infections in humans and animals. However, the uncontrolled use of antibiotics not only increases bacterial resistance, but also brings a huge burden to the ecological environment and poses a serious threat to the human living environment.

[0003] Common methods for removing tetracycline from water generally include biological treatment, adsorption, ozonation and advanced oxidation methods. Among them, the advanced oxidation method has a more obvious degradation effect on antibiotics. Advanced oxidation methods include photocatalytic oxidation, ozone oxidation, Fenton oxidation, persulfate oxidation and electrochemical oxidation. Photocatalytic oxidation is a pollutant degradation technology that uses semiconductors as catalysts and solar energy to decompose organic pollutants. Known photocatalysts include ZnO, TiO, WO3, g-C3N4, In2O3 and Ag3PO4. However, due to the limitations of high carrier recombination rate, poor stability and low light energy utilization, this type of photocatalyst has limited responsiveness to visible light and is greatly restricted in practical applications.

[0004] Ag3PO4 has strong oxidizing properties and can decompose organic compounds dissolved in water under sunlight. However, Ag3PO4 is slightly soluble in water, has poor stability, and is easily photoetched into silver during the photocatalytic process. The presence of these factors reduces the photocatalytic activity and structural stability of Ag3PO4, limiting its application in practical processes.

[0005] Methods to improve the photocatalytic performance of Ag3PO4 include controlling crystal morphology, surface engineering, ion doping, coupling with another semiconductor, etc. These methods all improve the photocatalytic activity of Ag3PO4 by accelerating the transport rate of carriers and promoting the separation of photogenerated electrons and holes. The construction of heterojunctions is an effective strategy to extend the carrier lifetime by adjusting the migration path of photogenerated electrons and holes in heterojunctions. This photocatalytic effect can promote the separation of photogenerated electrons and holes in Ag3PO4 to a certain extent, but its redox ability is still not ideal.

[0006] In2O3 is an n-type transparent semiconductor functional material with the characteristics of wide band gap, low resistivity and high catalytic activity. However, due to its high carrier recombination rate, its application in photocatalytic degradation of pollutants is greatly limited. Summary of the invention

[0007] The purpose of the present invention is to provide a photocatalyst for degrading doxycycline and a preparation method thereof, so as to solve the problem of antibiotic pollution caused by the use of doxycycline.

[0008] The object of the present invention is achieved in that:

[0009] A method for preparing a photocatalyst for degrading doxycycline comprises the following steps:

[0010] S1. Synthesis of MIL-68(In): Dissolve 120 mg of In(NO3)3·5H2O in 40 mL of N,N-dimethylformamide (DMF) and completely dissolve it in ultrasound. Then add 120 mg of terephthalic acid and dissolve it in ultrasound. React in a 120°C oil bath for 40 min, cool to room temperature, centrifuge and wash the precipitate three times with ethanol to obtain MIL-68(in).

[0011] Synthesis of S2, In2O3 / Ag3PO4: Dissolve 0.5g of MIL-68 (In) and 12~304mg of Ag3PO4 in 50mL of H2O to form solution A; dissolve 10~254mg of Na2HPO4 in 50mL of H2O to form solution B; mix solution A and solution B and stir for 2h, wash once with water and ethanol, and then vacuum dry at 60°C for 10h; place the dried product in a calcination furnace, and calcine at 400°C for 3h at a heating rate of 3°C / min to obtain the photocatalyst of the present invention - In2O3 / Ag3PO4.

[0012] The purpose of the present invention can also be achieved like this:

[0013] A photocatalyst for degrading doxycycline is prepared by adopting the above preparation method.

[0014] The present invention prepares and synthesizes a new type of photocatalyst In2O3 / Ag3PO4. By synthesizing the In2O3 / Ag3PO4 heterojunction photocatalyst, the existence of oxygen vacancies therein is proved, thereby maximizing the advantages of both, improving the response to visible light, revealing the mechanism of photogenerated charge migration, and realizing the effective separation of photogenerated carriers. The present invention studies the carrier separation efficiency, photostability and photocatalytic activity of the photocatalyst by taking doxycycline as the target pollutant. The results show that under visible light irradiation, the photocatalyst of the present invention has a high decomposition efficiency and good stability for doxycycline. The mechanism of photoinduced charge migration of the photocatalyst of the present invention is revealed by DFT calculation and experimental data analysis. The successful synthesis of the heterojunction greatly promotes the migration and separation of photogenerated electron-hole pairs, thereby improving its photocatalytic activity and completing the degradation of pollutants.

[0015] The present invention prepares In2O3 / Ag3PO4 heterogeneous photocatalysts with different mass ratios. The microstructure, surface morphology, chemical state and optical properties of In2O3 / Ag3PO4 are studied by means of SEM, TEM, XRD, FTIR, XPS and UV-vis. In addition, the photocatalytic activity of the In2O3 / Ag3PO4 heterojunction is studied by transient photocurrent response, EIS Nyquist spectrum and Mott-Schottky spectrum. The results show that the In2O3 / Ag3PO4 composite material has the best photocarrier generation and transport capabilities, and its photocatalytic activity is much higher than that of pure In2O3 and Ag3PO4. In addition, the photocatalytic degradation experiment shows that under visible light irradiation, when the mass ratio of In2O3 / Ag3PO4 is 1:2, the degradation rate of doxycycline is the largest, and the reaction rate of In2O3 / Ag3PO4 is the highest, which is 6.94 times and 4.7 times that of In2O3 and Ag3PO4, respectively. The research results of the present invention have guiding significance for the design and development of efficient visible light responsive photocatalysts and have market application prospects in environmental protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 are electron microscope images of various materials; among them, (a) is Ag3PO4, (b) is In2O3, (c) is In2O3 / Ag3PO4 (5:1), (d) is In2O3 / Ag3PO4 (2:1), (e) is In2O3 / Ag3PO4 (1:1), (f) is In2O3 / Ag3PO4 (1:2), (g) is In2O3 / Ag3PO4 (1:5), (h)~(i) are TEM micrographs of In2O3 / Ag3PO4 (1:2), and (j)~(n) are elemental maps of In2O3 / Ag3PO4 (1:2).

[0017] Figure 2 It is the XRD spectra of In2O3, Ag3PO4 and In2O3 / Ag3PO4 nanocomposites.

[0018] Figure 3 It is the chemical characterization diagram of In2O3, Ag3PO4 and the photocatalyst of the present invention; Among them, (a) is the FTIR diagram of In2O3, Ag3PO4 and the photocatalyst of the present invention; XPS spectra of the three nanocomposites, In2O3, Ag3PO4 and In2O3 / Ag3PO4: (b) is survey, (c) is O 1s, (d) is P 2p, (e) is In 3d, and (f) is Ag 3d.

[0019] Figure 4The optical characterization diagrams of In2O3, Ag3PO4 and the photocatalyst of the present invention; wherein, (a) is the UV-Vis diffuse reflectance spectrum (DRS) diagram; (b) is the band gap calculation of In2O3; (c) is the band gap calculation of Ag3PO4; (d) is the photoluminescence spectrum of In2O3, Ag3PO4 and the photocatalyst of the present invention; (e) is the nitrogen adsorption-desorption isotherm diagram; (f) is the pore size distribution diagram of the adsorption branch of the isotherm calculated by the Barrett-Joyner-Halenda (BJH) method.

[0020] Figure 5 These are the electrochemical characterization diagrams of In2O3, Ag3PO4 and the photocatalyst of the present invention; wherein, (a) is the electrochemical impedance spectroscopy (EIS) diagram; (b) is the transient photocurrent response (TPR) diagram; and (c) is the Mott-Schottky diagram.

[0021] Figure 6 It is a diagram of the photocatalytic degradation of doxycycline under visible light; wherein, (a) is a schematic diagram of the degradation of doxycycline in the presence of Ag3PO4, In2O3 and different In2O3 / Ag3PO4 nanocomposites; (b) is a schematic diagram of the quasi-secondary degradation kinetics of doxycycline in the presence of Ag3PO4, In2O3 and different In2O3 / Ag3PO4 nanocomposites; (c) is a schematic diagram of the photocatalytic degradation of doxycycline (30 mg / L) under visible light: the change in the photocatalytic activity of 1:2 In2O3 / Ag3PO4 after 5 consecutive cycles of use; (d) is a comparison of the XRD spectra of the original and reacted In2O3 / Ag3PO4. DETAILED DESCRIPTION

[0022] The present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0023] like Figure 1 As shown, the preparation method of the photocatalyst of the present invention comprises the following steps:

[0024] S1. Synthesis of MIL-68(In): Dissolve 120 mg of In(NO3)3·5H2O in 40 mL of DMF and dissolve completely in ultrasound. Then add 120 mg of terephthalic acid and dissolve in ultrasound. React in an oil bath at 120°C for 40 min, cool to room temperature, centrifuge, and wash the precipitate three times with ethanol to obtain MIL-68(in).

[0025] Synthesis of S2, In2O3 / Ag3PO4: 0.5 g MIL-68(In) +(12, 30, 61, 121, 304 mg) of Ag3PO4 was dissolved in 50 mL of water to form solution A; (10, 25, 51, 101, 254 mg) of Na2HPO4 was dissolved in 50 mL of H2O to form solution B. Solution A and solution B were mixed and stirred for 2 hours, washed with water and ethanol once each, and dried in vacuum at 60°C for 10 hours. The final product In2O3 / Ag3PO4 was obtained by calcining at 400°C for 3 hours at a heating rate of 3°C / min. (In2O3 / Ag3PO4 mass ratio: 5:1, 2:1, 1:1, 1:2, 1:5).

[0026] Photocatalytic experiment of the photocatalyst of the present invention:

[0027] The photocatalytic activity of the samples was evaluated by the degradation of doxycycline under visible light. A xenon light catalytic device was used as a visible light source. The initial concentration of doxycycline was about 30 mg / L. 100 mg of the photocatalyst was dispersed in 100 mL (30 mg / L) of the reaction solution. The mixed solution was rotated at 220 r / min for 30 min in the dark before illumination to achieve adsorption-desorption equilibrium. About 2 mL of the suspension was removed at 0, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 50 min and 60 min using a 2 mL plastic syringe with a needle, and then the catalyst particles were removed by a 0.22 μm flow filter membrane. The concentration of doxycycline was analyzed by UV-Vis spectrophotometer with a detection wavelength of 355 nm and a sample volume of 0.8 mL.

[0028] like Figure 1 As shown in Figure 1, the microstructures of pure In2O3, Ag3PO4 and the photocatalyst of the present invention, In2O3 / Ag3PO4, with different mass ratios were observed by SEM. Figure 1 As shown in (a), it is composed of irregular spherical particles with diameters between 500nm and 2μm. Figure 1 (b) shows that In2O3 has a hollow hexagonal prism morphology, which is consistent with previous reports. This morphology provides a wide space for the growth of Ag3PO4 to construct heterojunctions. Figure 1 (c)~ Figure 1(g) SEM images of In2O3 / Ag3PO4 (5:1), In2O3 / Ag3PO4 (2:1), In2O3 / Ag3PO4 (1:1), In2O3 / Ag3PO4 (1:2) and In2O3 / Ag3PO4 (1:5) samples. A large number of irregular spherical Ag3PO4 nanoparticles are attached to the surface of In2O3 in the form of hollow hexagonal prisms to form In2O3 / Ag3PO4 composite materials. When the mass ratio of In2O3 / Ag3PO4 changes from (5:1) to (1:1), the particle size of the sample gradually decreases, and when the mass ratio of In2O3 / Ag3PO4 changes from (1:1) to (1:5), the particle size of the sample gradually increases. Comprehensive reference Figure 1 (a)~ Figure 1 (g), it can be concluded that pure In2O3 and pure Ag3PO4 are successfully composited. Figure 1 (h)~ Figure 1 As shown in (i), TEM and HRTEM of In2O3 / Ag3PO4 (1:2) were analyzed and displayed to further confirm the construction of In2O3 / Ag3PO4 composite materials. Figure 1 It can be further observed that a large number of irregular spherical Ag3PO4 nanoparticles are attached to the surface of In2O3. Figure 1 (i) is the HRTEM image of In2O3 / Ag3PO4 (1:2). The lattice fringe spacing is 0.26nm and 0.29nm, corresponding to the Ag3PO4 (210) crystal plane and the In2O3 (220) crystal plane, respectively. The two lattices are in contact with each other, and the close contact is conducive to the transfer of carrier charges. It is intuitively confirmed that the construction of the heterojunction can lead to the recombination of the two types of semiconductors and produce a synergistic effect. In addition, Figure 1 (j)~ Figure 1 (n) The elemental mapping of the In2O3 / Ag3PO4 composite material is provided, confirming the uniform distribution of elements O, Ag, In and P in the In2O3 / Ag3PO4 composite material.

[0029] The crystal structures of pure In2O3, pure Ag3PO4 and In2O3 / Ag3PO4 were further verified by XRD. Figure 2As shown, the diffraction peaks of all samples are clear and distinct, indicating that the crystallinity of all samples is high. All peaks of pure In2O3 nanoparticles are very consistent with the peaks of well-crystallized In2O3 (JCPDS No.06-0416). The characteristic diffraction peaks of In2O3 at 2θ = 30.47°, 35.42°, 45.75°, 50.90° and 60.52° correspond to the (222), (400), (431), (440) and (622) planes, respectively. The prepared pure Ag3PO4 sample shows 10 diffraction peaks at 22.95°, 31.77°, 35.39°, 38.65°, 44.70°, 49.89°, 54.87°, 57.14°, 59.37° and 63.78°, which correspond to the (110), (200), (210), (211), (220), (310), (222), (320), (321) and (400) planes of the typical body-centered cubic structure Ag3PO4 (JCPDS No. 06-0505). The diffraction peaks of the prepared In2O3 / Ag3PO4 sample correspond to In2O3 and Ag3PO4. It is worth noting that the peaks of In2O3 / Ag3PO4 near 30.65°, 35.50°, 51.11° and 60.85° are caused by the overlap of the diffraction peaks of In2O3 and Ag3PO4. The diffraction peak intensity near 2θ=44.01° and 64.15° gradually decreases with the increase of In2O3 component, which may be due to the increase of introduction ratio and catalyst crystallinity. In addition, Figure 2 It can also be found that the XRD peak of In2O3 / Ag3PO4 moves to a low angle and no phase change occurs, indicating the presence of oxygen vacancies. The XRD spectrum of the In2O3 / Ag3PO4 composite material has no other impurity peaks except In2O3 and Ag3PO4, indicating that Ag3PO4 nanoparticles are successfully loaded on the In2O3 surface. At the same time, there is no chemical reaction between Ag3PO4 and In2O3, and the hybrid catalyst has high purity.

[0030] like Figure 3 As shown in (a), pure In2O3, Ag3PO4 and In2O3 / Ag3PO4 composites were characterized by FTIR to confirm the chemical structure of the composites. For pure In2O3, the wavelength range of 3240~3580cm -1 The broad peaks in the range correspond to the stretching and bending vibration modes of the OH groups of adsorbed molecular water. -1 Two peaks can be observed at 1073 cm-1, which is caused by the deformation vibration of the -OH group in the adsorbed water molecules. -1 539cm -1The significant peak at is the characteristic peak of In2O3 corresponding to the vibration of In-O-In bond phonon. For pure Ag3PO4, the broad peak is at 3240~3580cm -1 In the range, the significant peak is located at 1654cm -1 , which may be due to the OH stretching vibration and HOH bending vibration of water adsorbed on the sample surface. 1400cm -1 The absorption peak at 1073cm comes from the stretching vibration and harmonics of the P=O double bond. -1 Attributable to PO4 3- Asymmetric stretch properties. 861cm -1 The peak at belongs to PO4 3- PO stretching vibration mode. 548cm -1 The peaks near correspond to the in-plane bending vibration of the O=PO group. For In2O3 / Ag3PO4 composites with different composite ratios, overlapping peaks and characteristic peaks of the functional groups of In2O3 and Ag3PO4 can be observed, indicating the coexistence of the two components. The above results show that the In2O3 / Ag3PO4 composites were successfully prepared.

[0031] XPS was used to analyze the surface composition and chemical state of pure In2O3, Ag3PO4 and In2O3 / Ag3PO4 composites. Figure 3 (b) shows that there are elements such as O, In, Ag, C, and P in the In2O3 / Ag3PO4 composite material, indicating that the In2O3 / Ag3PO4 composite material is composed of In2O3 and Ag3PO4. Figure 3 (c)~ Figure 3 (f) High-resolution XPS spectra of O 1s, P 2p, In 3d, and Ag 3d are provided to show the differences in binding energies. Figure 3 As shown in (c), the O1s spectrum of the sample can be fitted into three peaks. The peak at 530.35 eV is attributed to O in In2O3 and Ag3PO4. 2- The high energy peak of 531.26 eV is usually formed by oxygen vacancy clusters. The peak of 532.54 eV can be attributed to the surface hydroxyl groups of In2O3 and Ag3PO4. The high-resolution XPS spectrum of P 2p ( Figure 3 (d)) can be decomposed into two peaks centered at 132.73 eV and 133.79 eV. In the 3D core-level XPS spectrum ( Figure 3 In (e), two strong characteristic peaks are clearly observed in the two samples. 444.43 eV corresponds to In 3d 3 / 2 , 451.98 eV corresponds to In3d 5 / 2 , indicating that In is 3+ Valence exists. Figure 3 (f) is the binding energy of Ag 3d, where the two peaks at 367.56 eV and 373.64 eV in the Ag 3d spectrum belong to Ag + Ag 3d 5 / 2 and Ag 3d 3 / 2 Compared with pure Ag3PO4, the O 1s, P 2p and Ag 3d binding energies of In2O3 / Ag3PO4 are reduced, indicating the formation of chemical bonds and the enhancement of the electron density of Ag3PO4. Based on the above analysis, it not only proves the existence of In2O3 and Ag3PO4 in In2O3 / Ag3PO4, but also further illustrates the close contact interaction between In2O3 and Ag3PO4.

[0032] The UV-Vis DRS of the prepared pure In2O3, pure Ag3PO4 and In2O3 / Ag3PO4 composites with different mass ratios were measured to study the light absorption characteristics of the photocatalyst. The results show that all samples have strong absorption in the ultraviolet and visible regions. The absorption band edge of pure In2O3 is around 550nm, which is narrow, limiting its photocatalytic performance. For pure Ag3PO4, the absorption edge is around 530nm, which is consistent with our previous report. When Ag3PO4 grows on the surface of In2O3, the light absorption capacity of the composite material is enhanced. Compared with monomer In2O3, the composite material has a significant red shift, which may be due to the interaction between In2O3 and Ag3PO4. With the increase of Ag3PO4 content, the absorption edge of the In2O3 composite material moves to longer wavelengths. According to the DRS spectrum, the band gap (Eg) of the obtained sample was estimated using the Kubelka-Munk equation:

[0033]

[0034] Where: Eg is the semiconductor band gap, A is the absorption constant, and v is the frequency of light (s -1 ), h is Planck's constant; the value of n is determined by the transition model of the semiconductor.

[0035] from Figure 4As can be seen from (b) and 4(c), the calculated band gaps (Eg) of In2O3 and Ag3PO4 are 2.65eV and 2.44eV, respectively, which is consistent with previous reports. The Eg value of the In2O3 / Ag3PO4 composite material is smaller than that of pure In2O3, indicating that the introduction of Ag3PO4 changes the performance of In2O3 through interaction. The separation of photogenerated charges is crucial to the photocatalytic activity of semiconductors, and luminescence is caused by the migration, transfer and recombination of photoluminescent carriers in semiconductors. Therefore, the luminescence spectrum is used to determine the separation efficiency of photogenerated carrier capture and migration separation in semiconductors. Generally speaking, the better the separation and migration efficiency of photoluminescent carriers, the lower the photoluminescence intensity in the luminescence spectrum, indicating that the recombination rate of photoelectron-hole pairs is lower and the semiconductor catalyst has higher photocatalytic activity. The luminescence spectra of pure Ag3PO4, pure In2O3 and In2O3 / Ag3PO4 (1:2) photocatalysts are shown in Figure 2. Figure 4 (d). Spectral analysis shows that the emission peaks of pure Ag3PO4, pure In2O3 and In2O3 / Ag3PO4 (1:2) are all located at 394 nm, and the emission peak range is 350-450 nm. In addition, the emission peak intensity of Ag3PO4 is the highest, indicating that the electron-hole pairs in Ag3PO4 are more easily recombined. Compared with pure In2O3 and Ag3PO4, the luminescence peak intensity of the In2O3 / Ag3PO4 composite material is significantly reduced, which may be due to the heterojunction structure formed in the In2O3 / Ag3PO4 composite material, which reduces the migration distance of photogenerated carriers and promotes the transfer and separation of photogenerated carriers. The results show that the heterogeneous structure greatly inhibits the recombination of photogenerated electrons and hole pairs, thereby improving the photocatalytic activity and performance yield. It is well known that specific surface area is an important factor affecting the evolution of photocatalytic performance. It is generally believed that catalysts with high specific surface area have higher photocatalytic activity. Therefore, the nitrogen adsorption-desorption isotherm ( Figure 4 (e) Brunauer-Emmet-Teller (BET) surface areas and their corresponding pore size distributions of In2O3, Ag3PO4, and In2O3 / Ag3PO4 nanocomposites. The mesopore volume of In2O3 / Ag3PO4 is 0.410173 cm 3 / g, higher than In2O3 (0.231061cm 3 / g) and Ag3PO4 (0.003375cm 3 / g). The Barrett Joyner Halenda (BJH) method was used to calculate the pore size distribution curve of the adsorption branch. The average pore size of the In2O3 / Ag3PO4 nanocomposite material is 18.3797nm, which is larger than that of In2O3 (16.6590nm) and Ag3PO4 (4.0815nm) ( Figure 4(f)). The BET surface areas of pure In2O3 and Ag3PO4 nanocomposites are 70.7744 and 41.9040 m 2 g -1 , while the BET surface area of ​​In2O3 / Ag3PO4 nanocomposite is 92.2458m 2 g -1 The hierarchical structure of In2O3 / Ag3PO4 with a higher specific surface area can provide more active sites for photocatalytic reactions, which is beneficial to the improvement of photocatalytic activity and provides an effective transfer pathway between reactants and products.

[0036] In order to obtain more information about the transition behavior and charge separation of the In2O3 / Ag3PO4 heterostructure, some photoelectrochemical measurements were performed. EIS can provide some information about the electron transport capacity of the sample. At the same time, the changes in EIS are related to the interfacial properties of the material. The presence of a built-in electric field facilitates the migration and separation of photogenerated charges at the interface, and one of the visual manifestations of this phenomenon is the reduction of impedance. Figure 5 As shown in (a), compared with pure In2O3 and pure Ag3PO4, the EIS Nyquist curve arc radius of the In2O3 / Ag3PO4 composite material is the smallest, indicating that it has the lowest electronic conversion resistance, the highest charge separation efficiency and the best photocatalytic performance. In addition, the transient photocurrent response of the sample under visible light irradiation was measured through 5 on-off intermittent irradiation cycles. It is well known that the photocurrent density curve can reflect the separation and migration ability of photogenerated carriers. From Figure 5 As can be seen in (b), in 5 consecutive on-off cycles, the transient photocurrent response of the sample presents a repeatable and relatively stable photocurrent curve. In addition, under the same test conditions, the transient photocurrent density of the In2O3 / Ag3PO4 composite material is significantly higher than that of pure In2O3 and pure Ag3PO4, indicating that the In2O3 / Ag3PO4 composite material has the best ability to generate and transport photocarriers. The semiconductor type and band structure of In2O3 / Ag3PO4 were further studied by the Mott-Schottky diagram. Figure 5As shown in (c), since the slope of the Mott-Schottky plot is positive, In2O3 and Ag3PO4 are both n-type semiconductors, and their flat band potentials are determined near their EF. The flat band potentials of In2O3 and Ag3PO4 at the intersection of the x-axis are ≈-0.78 and 0.44V (vs Ag / AgCl, pH≈7), respectively. According to the potential conversion relationship E(NHE) = E(Ag / AgCl) + 0.197, the corresponding CB values ​​are calculated to be -0.58V and 0.64V (relative to NHE, pH=7), respectively. Therefore, based on the above-mentioned band energies of In2O3 and Ag3PO4, combined with the Eg values ​​of In2O3 and Ag3PO4 obtained by UV-vis analysis, the formula is as follows:

[0037]

[0038] The valence band energies (EVB) of In2O3 and Ag3PO4 relative to NHE are +2.07V and +3.08V, respectively.

[0039] The photocatalytic activity of the prepared photocatalysts was studied using a xenon light catalytic device as a visible light source and doxycycline as the target pollutant. In addition, a blank experiment was performed to improve the accuracy of the experiment. All mixed solutions were rotated in the dark for 30 minutes before illumination to achieve desorption-adsorption equilibrium. The degradation effects of different samples are shown in Figure 2. Figure 6 (a). The results show that without adding photocatalyst, the concentration of doxycycline solution decreased slightly, indicating that doxycycline is relatively stable. The catalytic activity of pure In2O3 is not ideal. After 60 minutes of visible light irradiation, the degradation rate of doxycycline is only 27.0%. This may be due to the wide band gap of In2O3 and the narrow response range to visible light, resulting in its low photocatalytic efficiency. Due to the narrow band gap of Ag3PO4, the degradation rate of doxycycline reached 60.0% after 60 minutes of irradiation. Due to the e in a single catalyst - and h +Ag3PO4 is easy to reorganize under light conditions, and the photocatalytic activity of Ag3PO4 is low. Compared with pure In2O3, the composite material has better photocatalytic performance, and the degradation rate of doxycycline is above 40%. In2O3 / Ag3PO4 composite materials with different mass ratios show different degradation rates, and the order is In2O3 / Ag3PO4 (1:2)>In2O3 / Ag3PO4 (2:1)>In2O3 / Ag3PO4 (1:1)>In2O3 / Ag3PO4 (1:5)>In2O3 / Ag3PO4 (5:1). Under the same reaction conditions, when the ratio of In2O3 / Ag3PO4 is 1:2, the degradation rate reaches a maximum value of 80.8%, which is due to the formation of heterojunction and the increase of catalytic active sites. In addition, all photocatalysts used for photocatalytic degradation of doxycycline conform to the pseudo-second-order kinetic model:

[0040]

[0041] Where t is the irradiation time and k is the kinetic constant (min -1 ), C0 is the initial concentration of doxycycline at 0, and C is the actual concentration of doxycycline at t. The kinetic curves of different samples are shown in Figure 6 (b) As shown. The k constants of In2O3, Ag3PO4, In2O3 / Ag3PO4 (5:1), In2O3 / Ag3PO4 (2:1), In2O3 / Ag3PO4 (1:1), In2O3 / Ag3PO4 (1:2) and In2O3 / Ag3PO4 (1:5) were calculated by linear regression to be 0.00033min -1 、0.00048min -1 、0.00025min -1 、0.00137min -1 、0.00081min -1 、0.00229min -1 and 0.00049min -1 . The change in reaction rate is consistent with the change in photocatalytic activity. Among them, the reaction rate of In2O3 / Ag3PO4 (1:2) is the highest, which is 6.94 times and 4.7 times that of In2O3 and Ag3PO4 respectively, indicating that the formation of a heterojunction between In2O3 and Ag3PO4 is beneficial to improving its photocatalytic performance. The photostability and reusability of photocatalysts are another important factor affecting their practical applications. Under the same experimental conditions, through continuous degradation experiments, the changes in the catalytic performance of In2O3 / Ag3PO4 after five uses were obtained. Figure 6 As shown in (c), after 5 consecutive cycles, there is no obvious loss in the photodegradation effect of the In2O3 / Ag3PO4 composite material. Figure 6(d) is the XRD diffraction pattern of the In2O3 / Ag3PO4 composite material before and after the photocatalytic reaction. The XRD spectrum of the recycled In2O3 / Ag3PO4 composite material has no obvious change compared with the raw material, indicating that the In2O3 / Ag3PO4 composite material has good light corrosion resistance. The results confirm that the In2O3 / Ag3PO4 composite material has good reproducibility and stable crystal structure, and is considered to be a promising and effective environmental remediation photocatalyst.

Claims

1. A method for preparing a photocatalyst for degrading doxycycline, characterized in that: The following steps are involved: S1. Synthesis of MIL-68 (In): Dissolve 120 mg of In(NO3)3·5H2O in 40 mL of N,N-dimethylbenzene DMF and completely dissolve it in ultrasound; then add 120 mg of terephthalic acid and dissolve it in ultrasound; react in a 120°C oil bath for 40 min, cool to room temperature, centrifuge and wash the precipitate three times with ethanol to obtain MIL-68 (In); Synthesis of S2, In2O3 / Ag3PO4: Dissolve 0.5g of MIL-68 (In) and 12~304mg of Ag3PO4 in 50mL of H2O to form solution A; dissolve 10~254mg of Na2HPO4 in 50mL of H2O to form solution B; mix solution A and solution B and stir for 2h, wash once with water and ethanol, and then vacuum dry at 60℃ for 10h; place the dried product in a calcination furnace and calcine at 400℃ at a heating rate of 3℃ / min for 3h to obtain the photocatalyst - In2O3 / Ag3PO4.

2. A photocatalyst for degrading doxycycline, characterized in that: The photocatalyst is prepared by the preparation method described in claim 1.

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